This invention pertains generally to pulse Doppler radar receivers and more particularly to a receiver intended for use with a Fast Fourier Transform (FFT) processor without any need for automatic gain control.
As is known in the art, conventional pulse Doppler radar receivers, especially those intended for use in missile seekers, employ automatic gain control (AGC) loops for controlling the gain of such receivers in order to respond to the wide dynamic range of input signals that can result from electronic countermeasures (ECM), clutter, and multiple target environments. In such receivers input signals are successively passed through a bank of roughing filters (centered on the target Doppler frequency), an intermediate frequency (IF) amplifier strip with a so-called fast automatic gain control (AGC) loop and an analog-to-digital (A/D) converter with a limited dynamic range. If the AGC loop close-out frequency is greater than the bandwidth of the roughing filters, the AGC loop will effectively normalize all signal level variations existing at the output of the roughing filters.
Although a receiver with a fast AGC loop is effective in an ECM environment, a slow AGC loop is better suited for operation when a number of targets are present or a target is in clutter. In such situations and simultaneous presence of more than one target line in the roughing filter bandwidth will appear to a fast AGC loop as a signal level variation, and the AGC loop will respond to suppress the apparent signal level variations. Such action will result in crossmodulation distortion, causing signal spreading losses and creating false target lines. The net effect is often a loss of target track. On the other hand, although a slow AGC loop following the roughing filters is desirable to handle the multiple target and the target in clutter situations, a slow AGC loop is vulnerable to saturation and excessive recovery time problems imposed by most ECM threats.
From the foregoing it would appear that the optimum approach would be to employ a receiver without AGC but with a dynamic range wide enough to avoid saturation under any operating conditions.